This invention relates to a thermoplastic multi-layer sheet used for production of packaging materials and, more particularly, relates to a thermoplastic multi-layer sheet having a foam core layer and a multi-resin layer preferably made with waste materials reclaimed from the multi-layer sheet manufacturing process.
Thermoplastic foam sheet materials, such as those made from polystyrene foam, have been used for packaging applications. These foamed sheet materials are typically made by extrusion using a blowing agent wherein polystyrene resin, for example, is heated in an extruder and is mixed in the extruder with a gaseous or volatile blowing agent and then extruded through a die and allowed to expand to form a polystyrene foam sheet. These foam sheets can be extrusion coated, for example, by the process described in U.S. Pat. No. 3,616,020 or U.S. Pat. No. 3,669,794, to add a coating or coatings to produce multi-layered foam sheets. White, U.S. Pat. No. 3,396,062, discloses a process for molding a composite foam article.
Multi-layer sheets made with various thermoplastic polymers have also been used extensively in packaging applications. Typically, these multi-layer sheets are produced by coextrusion of various plastic resins. In such multi-layer sheets, layers comprising a thermoplastic resin having oxygen barrier properties have been added to provide improved properties to the sheet. Peterson, U.S. Pat. No. 3,524,795, teaches a multi-layer packaging material employing polyolefin outer layers with an inner layer of saran which functions as an oxygen barrier layer. Other multi-layer sheets have used other resins, such as an ethylene vinyl alcohol copolymer or a polyamide, as the oxygen barrier resin. For example, Schirmer, U.S. Pat. No. 4,848,187, discloses a multi-layer sheet having a polypropylene or polyethylene outer layer, an ethylene vinyl alcohol copolymer ("EVOH") or vinylidene chloride copolymer oxygen barrier layer, an unsaturated vinyl ester bonding layer for adhesion to polystyrene, and an outer layer of polystyrene, foamed or unfoamed, adjacent the bonding layer. Nakabayashi, U.S. Pat. No. 4,576,995, discloses a multi-layer sheet with an EVOH oxygen barrier resin layer laminated between polystyrene outer layers with adhesive layers of a modified ethylene vinyl acetate copolymer. Japanese Patent Application 84012467, published Mar. 23, 1984, discloses a container made from a multi-layer structure comprising an outer polyethylene layer, an EVOH oxygen barrier resin layer, and an outer layer of one of polypropylene, nylons, polyesters, polycarbonates, polystyrenes and polyvinyl chlorides.
The production and use of these multi-layer materials often result in considerable amounts of scrap material being formed during the manufacturing operations. Accordingly, much effort has been directed towards the recycle of scrap material produced during the production of thermoplastic multi-layer sheets. However, scrap from these multi-layer materials is difficult to recycle because of the different thermoplastic compositions contained in the various layers. The different thermoplastics used often are incompatible with each other, resulting in the mixture of such thermoplastics having poor mechanical properties, such as impact strength, tensile strength, heat deflection temperature and elongation at break. For example, ethylene vinyl alcohol copolymer and polystyrene are immiscible, making recycle of scrap containing these two thermoplastics difficult.
One approach to avoid the incompatibility problem of thermoplastics contained in scrap is to blend a compatibilizer, such as a styrene-ethylene-co-butylene-styrene block copolymer like Kraton sold by Shell, with the scrap mixture. A compatibilizer, however, may not be available for the thermoplastics used. See The International Packaging Letter, Vol. 6, Number 516, May/June 1989 at p. 2, which states that three commercially available compatibilizers did not work for polystyrene and ethylene vinyl alcohol copolymer scrap.
Haefner, U.S. Pat. No. 3,940,001, and Schenk, U.S. Pat. No. 3,977,153, each disclose recycling of multi-layer structures wherein the individual layers are first separated to produce single component materials. This approach is difficult for structures having many layers and requires identification of what is in each layer.
Wallace, U.S. Pat. No. 4,647,509, describes a multi-layer laminate structure having a layer made of scrap from the multi-layer structure manufacturing process and a barrier layer, wherein the scrap layer is a blend of the scrap material produced during the manufacturing process with a quantity of an unused, or virgin, resin which makes up one of the other layers of the structure. Wallace does not disclose foam multi-layer sheets having a thermoplastic foam layer with an adjacent multi-resin reclaim layer.
An example of a compatibilizer approach for recycling multi-layer sheets is Catte, U.S. Pat. No. 4,234,663, which describes a multi-layer foil structure which comprises a thermoplastic styrenic polymer layer and a thermoplastic olefin polymer layer bonded by an intermediary polymer layer comprising at least one layer comprising a bonding agent wherein at least one layer containing scrap produced from the multi-layer foil structure is included intermediate of the styrenic polymer and olefinic polymer layers and the scrap containing layer comprises a mixture of scrap with a graft copolymer compatibilizer in a ratio of graft copolymer to scrap of 0.2:1 to 9:1. The graft copolymer compatibilizer is described further as being a polyolefin substrate with a styrene or styrenic monomer grafted onto the polyolefin. The structures of Catte are disclosed as manufactured by a coextrusion process or by extrusion coating. Catte does not disclose foam multi-layer sheets having a thermoplastic foam layer with an adjacent multi-resin layer, nor does he disclose recycling of multi-layer sheets comprising an oxygen barrier resin. He also does not disclose a multi-resin layer comprising an oxygen barrier resin. Further, he uses the graft copolymer compatibilizer in his reclaim layer.
Bonis, U.S. Pat. No. 4,402,889, describes a process to produce a multi-layer plastic sheet comprising outer layers of "qualified" scrap material, defined as either unused virgin resin or a scrap material consisting of only one resin, and a central layer of "unqualified" scrap material having a sufficient polyolefin content to provide a moisture barrier. Bonis uses a coextrusion process wherein separate streams of the qualified scrap material and the unqualified scrap material are coextruded together through the same die to produce the multi-layer sheet.
Hungerford, U.S. Pat. No. 4,287,147, describes a process for reusing scrap material resulting from the production of a laminated film made from a polyacrylonitrile homopolymer or interpolymers. This process involves redissolving the scrap in a solvent for reuse.
Benge, U.S. Pat. No. 4,652,325, describes a method for making a multi-layer plastic structure having a substantial asymmetry in the layers of the structure, which comprises two separate coextrusions to produce two multi-layer sheets followed by combination of the two sheets to produce the structure. The two sheets are combined by being brought into contact with each other at a time when at least one contact surface of one of the sheets is still in a fluid state. In the Benge method, reclaim material is used in the coextrusion steps to produce a layer in one of the sheets. Benge, U.S. Pat. No. 4,808,482, is a division of 4,652,325, and is directed to an asymmetrical multi-layer structure produced by the process of 4,652,325. The structure is disclosed as having at least six layers: two skin polymer layers surrounding a barrier resin layer which makes up one of the sheets and two additional skin polymer layers surrounding a reclaim material layer which make up the second sheet.
Kamoda, U.S. Pat. No. 4,410,602, describes a process for producing a multi-layer laminated resin film using reclaimed resin mixture in a coextrusion process wherein a layer of the reclaimed resin mixture is entirely surrounded by a fresh resin layer. In Kamoda, the reclaimed synthetic resin mixture surrounded by fresh resin layers is used as a top and bottom laminate with an interlayer interposed between these two laminates. The fresh resin used by Kamoda is disclosed as having a decomposition temperature higher than that of the reclaim resin mixture.
Briggs, U.S. Pat. No. 4,705,708, describes a multilayer laminate structure having a separate layer of scrap produced from several thermoplastic resins used in making the laminate structure, which is used to produce parisons for blow-molding applications.
Canadian Patent 996,018 discloses a reprocessible multi-layer laminate for food containers comprising a blend of high-density polyethylene, ethylene copolymer adhesives and polyamides wherein one layer consists of a melt homogeneous blend of polyethylene and scrap from the reprocessible laminate.
Brinkman, U.S. Pat. No. 4,013,745, describes a polyvinyl glass film extrusion process involving use of recycle material wherein the recycle material is returned to the extruder feed.
Thiel, U.S. Pat. No. 4,086,045, describes apparatus for producing thin-walled articles comprising extrusion to produce a web of thermoplastic material. Residue from the overall process is fed back to the extrusion stage where it is mixed with fresh raw material.
An Eval Company of America (EVALCA) technical information bulletin entitled "Technical Information on GF-20" (precise publication date is unknown, but a copy was obtained by Applicants in April, 1989), states that scrap material containing EVOH has been used in multi-layer structures and discloses a method to avoid processing anomalies during extrusion of reclaim containing polypropylene and EVOH, which comprises addition of GF-20, an additive sold by EVALCA.
Reclaim material consisting of polystyrene, impact polystyrene and non-thermoplastic printing inks has been mixed in small amounts with fresh impact polystyrene and used in the extrusion coating of an impact polystyrene layer on a polystyrene foam core. The same reclaim material has also been mixed in small amounts with fresh polystyrene and used in conventional extrusion of a polystyrene foam. This reclaim material does not comprise a thermoplastic oxygen barrier resin or polyolefin.
None of the previously disclosed uses of thermoplastic reclaim material in the production of multi-layer structures have used a multi-resin layer comprising an oxygen barrier resin as an outer layer. None of the previously disclosed processes have involved extrusion coating of a thin layer of multi-resin material comprising an oxygen barrier resin onto a foam core layer. None of the prior art processes or structures have involved the combination of an oxygen barrier film as one outer layer with a foam core layer and a second outer layer comprising multi-resin material. Further, none have addressed recycle of a mixture of polystyrene, polyolefin and an EVOH oxygen barrier resin.
It is accordingly an object of this invention to provide an improved thermoplastic multi-layer structure comprising a thermoplastic foam core layer, particularly a polystyrene foam core layer. It is another object of this invention to provide a thermoplastic multi-layer sheet capable of being processed into cups and trays or other packaging articles comprising a thermoplastic oxygen barrier layer laminated onto a foam core layer and at least one multi-resin layer comprising waste material reclaimed from the multi-layer sheet manufacturing process. It is yet another object to provide a recycle method for waste material comprising polystyrene, polyolefin and ethylene vinyl alcohol copolymer. Other objects will appear below.
We have found that the objects of the invention can be attained by a thermoplastic multi-layer sheet comprising a foam polystyrene layer and a multi-resin layer comprising a mixture of at least three thermoplastic components: a polystyrene, a polyolefin, and an ethylene vinyl alcohol copolymer. Such a multi-layer sheet is lightweight, strong, readily thermoformable into cups, containers and packages, and readily produced at commercially feasible rates.